The present invention is directed to monitoring data transmissions over networks, and in particular, to methods for monitoring data transmissions over asymmetric networks.
Communications networks, such as the internet typically support applications such as telephony. In internet telephony, data is transmitted typically in the form of packets. The delay associated with packet transmission is known as a propagation delay. This propagation delay is typically measured by round trip delay and may be used in order to optimize network utilization.
For example, FIGS 1 and 2 illustrate an asymmetric network 20 having nodes A and B and data transmission between these nodes. In FIG. 1, the network is capable of transmitting 5 Kbit/sec from node A to Node and 3 Kbit/sec from node B to node A (indicated by solid line arrows 21).
When additional data is desired to be transmitted from node B to node A, for example, at 6 Kbit/sec, the network becomes overloaded, as it is only capable of transmitting at 3 Kbit/sec from node B to node A. As a result, packets sent from node B to node A become delayed, since they can not be transmitted in the proper time Each new packet sent from node B (indicated by broken line arrow 22) will wait at node B and will be transmitted only after all other previously sent packets. Transmission delay grows to a point where all intermediate buffers at node B become full and node B starts to drop packets, resulting in packet loss.
To overcome the problem of transmission loss, the network should be monitored to detect such a situation. This is typically done by monitoring the round trip lime (RTT) for each packet. For example, RTT may be measured at node A by an algorithm that typically: 1) sends a probe packet from Node A to Node B at time A1; 2) Node B receives this packet at time B1; 3) Node B sends a reply at time B2; and 4) node A receives a reply to the packet sent at (1) at time A2. RTT is then calculated in accordance with the equation:
RTT=(A2-A1)xe2x88x92(B2-B1)
This relation is expressed graphically as shown in FIG. 3. Resultant RTT graphs for FIG. 1 are shown in FIG. 4 and for FIG. 2 are shown in FIG. 5. In FIG. 4, RTT is constant, as the network is able to handle the packet transmissions. This is not so for FIG. 5, where RTT increases due to network overload and back-up of packets.
RTT measured at node B behaves similarly to that measured at node A. This may be problematic when applied to asymmetric networks. As exemplified in FIG. 2, there is a problem in packet transmission from node B to node A, but not from node A to node D. If measuring RTI, a network load problem will be detected at both Nodes A and B, when this is not the case, since the network load problem is only at node B.
This problem is overcome by measuring one-way delay. However, measurement of one-way delay exhibits drawbacks in that clocks at nodes A and B must be synchronized in accordance with protocols such as RFC 1305 Network Time Protocol as detailed in Mills, xe2x80x9cNetwork Time Protocol (Version 3) Specification, Implementation and Analysisxe2x80x9d, Network Working Group, Request for Comments: 1306 (March 1997). This protocol is complicated as it requires distributed participation of a number of client/servers or peers arranged in a dynamically reconfigurable, hierarchically distributed configuration.
The present invention overcomes the problems associated with the conventional art by providing a simplified monitoring parameter based on relative one way delay between nodes in a network This measurement of relative one way delay does not require synchronization of the clocks with respect to each other at nodes A and B. As a result, of the present invention bitrate and bandwidth can be controlled with greater accuracy when compared to that of the conventional art.
In one aspect of the present invention, there is provided a method for measuring delay parameters in a packet switched network between node A and node B. Each node has a clock, which is not synchronized with the other. This delay parameters measuring method includes the steps of measuring a first relative one way delay (ROWD1) from node A to node B, measuring a second relative one way delay (ROWD2) from node A to node B and calculating the difference between the first and second relative one way delays for providing a relative one way delay (ROWD) parameter. The ROWD is provided by the following equation: ROWD=ROWD2xe2x88x92ROWD1.
In this manner, the stop of determining ROWD1 includes the steps of, node A sending to node B a first probe packet at a time A1, node B receiving the first probe packet at time B1, and calculating ROWD1 as ROWD1=R1xe2x88x92A1. Preferably, the step of determining ROWD2 includes the steps of: node A sending to node B a second probe packet at a time A2, node B receiving the second probe packet at time B2 and calculating ROWD2 as ROWD2=B2xe2x88x92A2.
In another aspect of the invention, an absolute parameter of one way delay (OWD) is calculated by subtracting a clock difference (CD) between the clocks of node A and node B from the ROWD and provided by the following equation:
OWD=ROWDxe2x88x92CD.
In another aspect of the present invention, there is provided a method for detecting congestion in a packet switched network by measuring xcex94ROWD The method includes steps associated with the delay parameters measuring method, detailed above, and the steps of, estimating a smoothed ROWD average An, estimating a smoothed ROWD Jitter Jn and comparing the last measurement of ROWD to a dynamic threshold which threshold is a function of An and Jn.
Preferably, the step of estimating An is according to the following equation:             A      n              -        g              ⁢                  A                  n          -          1                    ·              (                  15          16                )              +      ROWDn    ·          1      16      
Preferably, the step of estimating Jn is according to the following equation             J      n              -        t              ⁢                  J                  n          -          1                    ·              4        5              +            "LeftBracketingBar"              Δ        ⁢                  xe2x80x83                ⁢                  ROWD          x                    "RightBracketingBar"        ·          1      5      
Preferably, said dynamic threshold Td is vary according to following equation;
Td=Anxe2x88x921+3Jnxe2x88x921
In another aspect of the invention congestion in the network is detected by analyzing growth of the ROWD in comparison to Td.
In still another aspect of the invention, there is disclosed a method or improving bit rate control by Transmission Control Protocol(s) (TCP) in accordance with a relative one way delay (ROWD) parameter. The method includes steps associated with the delay parameters measuring method, detailed above, and the step of estimating the retransmission timeout value according to the last measured ROWD.